Introduction to the Trimerophytes

A number of fossil vascular plants of the
Devonian are
superficially similar to the "rhyniophytes" but show some features not
found in the rhyniophytes. These plants are grouped together in the
Trimerophytophyta, the "trimerophytes." This group is almost certainly
paraphyletic; that is, it does not include all the descendants of
a common ancestor. Nonetheless, trimerophytes are of great evolutionary
interest, because they include fossils that are close to the common ancestry of
prominent plant taxa, such as ferns,
progymnosperms, and
sphenophytes.
The trimerophytes appear to form a sort of basal group close to the ancestry
of these major taxa.

Like the rhyniophytes, trimerophytes lacked leaves and roots; most of the
plant body consisted of branching stems that were photosynthetic throughout
their length. Vascular tissue was present, forming a solid central bundle in
the center of the stem, or protostele. However, whereas rhyniophytes
branched dichotomously -- stems always branched into two equal
branches -- trimerophytes branched pseudomonopodially, which is a
way of saying that branching was unequal, forming a main stem, or axis,
with several smaller lateral branches. You can see this pattern of
branching in the picture of Psilophyton forbesii, at right. Lateral
branches typically branched dichotomously, and were often shortened to form
bushy "webs" of small, closely spaced branches. Some trimerophytes also bore
enations -- small flaps of tissue lacking vascularization, and therefore
not true leaves -- on the main stems, giving them a superficially
"thorny" appearance.

Like the rhyniophytes, but unlike the superficially similar
zosterophylls
from the same time period, trimerophytes bore sporangia at the tips of
branches. The spindle-shaped sporangia produced only
one type of spore: trimerophytes were thus homosporous. Spores
released from the sporangia would have germinated into gametophytes,
but no fossil trimerophytes gametophytes have been identified, and we do
not know whether trimerophytes, like their later relatives the ferns,
sphenopsids, and seed plants, had small, inconspicuous gametophytes.

Trimerophytes varied in size from a few centimeters to nearly a meter tall;
large trimerophytes were among the largest plants of the Early Devonian. One
of the meter-high trimerophytes, Pertica quadrifaria, has
attained some post-extinction fame as the state fossil of Maine. Another
species in this genus is pictured below.

Source:
Stewart, W.N. and Rothwell, G.W. 1993. Paleobotany and the Evolution of
Plants. Second edition. Cambridge University Press, Cambridge.